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Descriptions

Realistic assessment of the vertical distribution of clouds, particularly the occurrence
of multi-layered systems, is critical for accurate calculations of radiative transfer in
general circulation models. Such information is also useful in the design and improvement
of satellite retrieval techniques. Current methods for retrieving cloud properties
from satellite data assume that the clouds reside in single-layered systems. These methods
are not expected to be successful for multi-layered systems.
Attempts to specifically address the question of cloud layering have thus far been
limited, due in part to the difficulties of inferring vertical cloud structure from either
surface or satellite data. In situ observations, such as those provided by aircraft, are
available only for localized regions and are limited in time. This study uses data from
a lidar instrument flown onboard the space shuttle and satellite imagery data to identify
the frequencies of occurrence of layered cloud systems at different spatial scales over
various regions of the globe.
The Lidar In-Space Technology Experiment (L1TE) was flown on Space Shuttle
Discovery in September 1994 and provided global-scale, high vertical resolution profiles
of the earth's troposphere and lower stratosphere. Analysis of the LITE observations
requires distinguishing clouds residing in organized, well-defined layers from clouds that
are distributed in altitude throughout the troposphere. The analysis employs a histogram
technique in which peaks having some critical number of observations are considered to
correspond to observations belonging to well-defined cloud layers.
Advanced Very High Resolution Radiometer (AVHRR) data for the 11-day duration
of the LITE mission are analyzed using the spatial coherence method. This method
identifies regions of locally uniform emission which are associated either with cloud-free
pixels or with overcast pixels corresponding to clouds in a single layer at a well-defined
altitude. The number of layers present is determined by the number of overcast radiances
associated with pixel arrays exhibiting locally uniform emission within the region.
Layer statistics are compiled for the Pacific, Atlantic, and Indian Oceans and the
North and South American, African, European, Asian, and Australian continents using
horizontal scales of 60 and 250 km. The results indicate a strong dependence on the
spatial scale chosen for the analysis, with two- and three-layered systems more prevalent
at the 250-km scale. Analysis of cloud-top altitudes from LITE and AVHRR show
that low-level cloud systems comprise the majority of the observations over both ocean
and land.